1. Field
The embodiments relate to DQPSK optical receivers, and more particularly, to a DQPSK optical receiver for receiving and demodulating a Differential Quadrature Phase Shift Keying (DQPSK)-modulated optical signal.
2. Description of the Related Art
In recent years, there has been an increasing demand for introduction of next-generation 40-Gbit/s optical transmission systems. Next-generation optical transmission systems are required to provide transmission distance and frequency utilization efficiency equivalent to those achieved by 10-Gbit/s optical transmission systems.
As a means to implement next-generation optical transmission systems, novel modulation schemes are currently under study, and the adoption of DQPSK modulation scheme is actively investigated because this scheme is superior to conventional modulation schemes such as the NonReturn-to-Zero (NRZ) scheme in that the spectral width of the signal is half and also in respect of frequency utilization efficiency, chromatic dispersion tolerance, and device transparency.
A receiver of an optical transmission system employing the DQPSK modulation scheme is equipped with a pair of Mach-Zehnder interferometers associated with A (I) and B (Q) arms, respectively. Each Mach-Zehnder interferometer has an optical delay of τ corresponding to the symbol time of the optical transmission system. Also, to provide an optical phase difference between the two interferometers, the phases of the A and B arms are set to “π/4” and “−π/4”, respectively. Two output terminals of each interferometer are connected to an optical receiving circuit for performing optical-electrical conversion, and the optical receiving circuit is followed by a clock and data recovery circuit for distinguishing between “0” and “1” of the electrical signal supplied thereto.
In this receiver, in order to secure the required signal characteristic, the optical phase difference between the two interferometers needs to be set with accuracy, that is, the phases of the two arms need to be accurately controlled to “π/4” and “−π/4”, respectively. To meet the requirement, it is essential to employ feedback control. As one of such feedback control techniques, feedback control has been known which uses branch signals derived from the stage succeeding the A- and B-arm optical receiving circuits as well as branch signals derived from the stage succeeding the A- and B-arm clock and data recovery circuits (see, e.g., Japanese Unexamined Patent Publication No. 2007-20138).
In DQPSK optical receivers, individual parts are often fabricated as modules to facilitate the maintenance and management of the receivers. In FIG. 1 of Japanese Unexamined Patent Publication No. 2007-20138, for example, it is conceivable that the balanced detectors 110 and 113 are unified into a module and that the clock and data recovery circuits 111 and 114 are also unified into a module.
The receiver architecture generally differs from equipment. For example, in FIG. 1 of Japanese Unexamined Patent Publication No. 2007-20138, the clock and data recovery circuits 111 and 114 may be incorporated into signal processing circuits succeeding the clock and data recovery circuits so that the clock and data recovery circuits and the signal processing circuits may constitute a single module.
To reduce the cost and size of receivers, branch points for the feedback control (points where the signals output from the optical receiving circuits and those output from the clock and data recovery circuits are branched) should preferably exist in a single unit. It is conceivable, therefore, that the feedback signals are derived from points other than the output side of the clock and data recovery circuits.
For example, in FIG. 1 of Japanese Unexamined Patent Publication No. 2007-20138, the mixer 120 may be input with the signal 124 instead of the signal 125, and the mixer 116 may be input with the signal 128 instead of the signal 129. This makes it possible for the branch points for the feedback control to exist in the same module comprising the balanced detectors 110 and 113. Even in cases where the clock and data recovery circuits 111 and 114 are incorporated into a separate module, leads for the feedback signals can always be extended from the module of the balanced detectors 110 and 113.
Where the signals not derived from the clock and data recovery circuits are used as the feedback signals, however, the feedback signals become deviated from optimum phase of the interferometers, giving rise to a problem that the signal characteristic is deteriorated.